什么时候将接口用作键类型对HashMaps有意义？

Question

It seems to me that any scenario where I instantiate a HashMap using an interface for a key-type would work contrary to the idea of the map. Take the following example:

HashMap<MyInterface, Integer> map = new HashMap<MyInterface, Integer>();

If I put 2 objects of different MyInterface implementations into the map then they will both operate on their own hashCode() implementations and the entire underlying logic of the HashMap no longer makes sense. So my question is, is there ever a proper time for using interfaces as keys in HashMaps ?

Answer 1

they will both operate on their own hashCode() implementations and the entire underlying logic of the HashMap no longer makes sense

I guess by that you meant hash collision. Hash collision does not corrupt the hash map. In java, you need to properly implement "equals" together with "hashCode" for the hash map to resolve the key.

Answer 2

How about:

Map<Connection,AtomicInteger> outstandingQueries = new HashMap<>();

A java.sql.Connection is an interface and this Map makes sense. Implementation of equals and hashCode can still distinguish objects of different types.